The goal of this proposal is to better understand the RNA metabolism of simple avian retroviruses (Rous sarcoma virus and avian leukosis viruses), which do not encode regulatory proteins. Our focus is on post-transcriptional regulatory events affecting the full-length unspliced viral RNA, which plays multiple roles as the packaged genomic RNA, the mRNA for the Gag and Pol proteins, and the pre-mRNA for env and src sub-genomic mRNAs. We will study the time period, beginning after the RNA is transcribed and ending when it is packaged into viral particles. We will investigate 1) the mechanism by which the single viral RNA transcript is spliced incompletely, maintaining a major population of unspliced RNA; 2) the means of export of the unspliced RNA from the nucleus to the cytoplasm and its localization in the cytoplasm; 3) the means of viral RNA stabilization and degradation. We will ask whether there are separate pools of unspliced RNA used exclusively for mRNA and genomic RNA or whether the same RNA can be first translated and then packaged. We will focus on cis-acting viral RNA elements characterized during previous funding periods of this grant: these include a negative regulator of splicing (NRS) element in gag, the direct repeat (DR) sequences flanking src, and a putative RNA stabilizing element in pol. In addition, we will seek to find new cis-acting elements necessary for nonsense-mediated decay to occur in the absence of splicing. Finally, we will also study the mechanism of lymphomagenesis by insertional mutagenesis with ALV mutants, which exhibit elevated readthrough and splicing to a downstream myb gene.